I think a closed-back GH2 would be very hard to tune even remotely similar to the open back version. I can hold my hand 1cm away from the grills on mine while listening and not only notice midrange reflections, but the midbass changes. If you completely cover the hole with something soft like a napkin (pourous?) it seems to decrease the punch they are known for. If I completely block them off with a solid drink coaster, they sound like absolute dog-spit. In order to sound decent I think the cups would need to be huge with a ton of damping, which would mean more weight, more expensive Cocobolo, and a hefty price tag. Maybe a GH5 and be a mix of the limited edition Heritage series and the Statement series? Or perhaps they could make a similar looking closed version with a different driver that has a different Qts and VAS.
EDIT: Also, I think the bushmills version is semi-open somewhere. Sorry didn’t mean to be a Debby Downer.